1. Field of the Invention
The present invention relates generally to a method of transporting solid particles pneumatically. More specifically, the present invention relates to method for pneumatically transporting material in a form of solid particles in a diluted phase through a conduit in which a varying material load arises by the feeding of the particles into the conduit. The particles are driven along the conduit with the aid of an air flow delivered by a pressurized-air source located upstream of a material infeed position.
The invention also relates to a corresponding plant system for the pneumatic transportation of material in a form of solid particles in a diluted phase through a conduit. The conduit includes a material infeed device, and a pressurized-air source upstream of the infeed device.
2. Related Art
The transportation of solid particles, such as wood chips, for instance, through a tubular conduit with the aid of a pressurized gas flow, particularly a compressed-air flow, is well known to the art. In one specific transport situation, the plant is dimensioned so that the source of pressurised-air will generate a specific air velocity in the conduit at a chosen maximum load on the system. The plant is dimensioned on the basis of the density of the bulk material (which in the case of wood chips will depend on the type of wood concerned and its moisture content), the concentration of the bulk material, and particle size distribution (chips, bark, etc.).
Earlier known transport systems have therefore been designed to manage the worst conceivable case, i.e. the case of unfavourable size distribution, a high moisture content, the conceivably heaviest bulk material and highest bulk concentration. On this basis, the source of pressurised-air has been designed to generate an air velocity which will ensure correct pneumatic transportation of the solid particles in said conceivably xe2x80x9cworse casexe2x80x9d with a chosen safety margin. Should the air velocity in the conduit fall beneath a critical value with respect to the volume of material concerned fed into the conduit, the particle material will settle in the conduit and normally necessitate emptying the conduit system mechanically/manually before the transport system can be restarted. A consequence of this nature is, of course, very serious and consequently it has been elected in respect of known systems to operate the pressurised-air source in a manner to maintain a constant free air flow from said source. The source of pressurised-air is normally referred to as a blower, i.e. a positive displacement compressor, which is operated at a constant speed.
One drawback with pneumatic transport systems of the type indicated above is that they have a relatively high energy consumption.
Accordingly, one object of the present invention is to provide a method and plant which enable the energy consumption of the system to be restricted when the system is operated with a varying material load. Another object is to provide a simple method of adjustment which enables this reduction in energy consumption to be achieved. Another object is to provide simple means by means of which the pressurised-air source can be adjusted to enable said reduction in energy consumption to be achieved.
One or more of these objects is achieved totally or partially with a method by which material in a form of solid particles in a diluted phase is pneumatically transported through a conduit. A varying material is established by feeding the particles into the conduit in a varying amount and driving the particles along the conduit with aid of an air flow delivered by a pressurized-air source located upstream of a material infeed position, and a transport air velocity for a maximum material load on air flow.
On or more of these objects is achieved totally or partially with a plant by which material in a form of solid particles in a diluted phase is pneumatically transported through the conduit. The conduit includes a material infeed device, and a pressurized-air source upstream of the infeed device.
Further developments of the invention will be apparent from the method according to the step of adjusting the free flow of the transport air source to bring the real value of the velocity of the transport air flow or the detected state into line with the control value, to establish a transport air velocity which is essentially equal to the chosen transport air velocity for the maximum material load, even in those cases when material loads are lower than the chosen maximum material load.
The method further includes the steps of establishing a relationship which is essentially independent of the material load for the chosen transport air velocity and as a relationship between the free flow of the transport air source and a pressure of the transport air flow, and sensing the pressure in the transport air conduit.
The pressurized-air source includes a positive displacement compressor, wherein the compressor has a rotary pump element, and wherein the free flow of the pressurized-air source is regulated by regulating a rotary speed of the compressor, which draws in air at ambient pressure.
The method further includes the steps of driving the rotary pump element of the compressor with an aid of an asynchronous motor whose speed is dependent on a frequency of the supply current to the motor, regulating the frequency to regulate the free flow of the pressurized-air source, and establishing a relationship between the control pressure and the frequency.
The method includes the step of correcting the relationship on a basis of reading off pump curves for the compressor.
Additionally, the method comprises the step of correcting the relationship with respect to air flow temperatures upstream and downstream of the compressor.
In the plant system of the present invention, a pressurized air source includes a positive displacement compressor and a rotatable drive shaft. The free flow delivered by the compressor varies linearly with a rotary speed. In that sensing member is for sensing a transport air pressure, and a relationship is established between the rotary speed of the compressor and the sensed pressure.
The compressor is driven by an asynchronous motor whose speed varies linearly with the frequency of a motor supply current. A frequency converter is connected between the asynchronous motor and a power source, and a frequency converter is controlled by a control member on a basis of a sensed pressure, wherewith a relationship is established between frequency and pressure, and the free flow of the pressurized air source corresponds to a frequency generated by the frequency converter.
The pressure can be measured readily and safely in the transport conduit and is dependent on the material load. Consequently, it is not necessary to measure the varying amount of material fed into the conduit. A simple relationship can be established between the free flow from said pressurised-air source and the detected/sensed pressure under certain conditions, such that the velocity of the transporting air will be constant with respect to different occurring material loads.
The free flow from said pressurised-air source is proportional to the rotary speed of a positive displacement blower used as said source. The rotary speed of the blower corresponds to the driving frequency of an asynchronous motor used to operate the blower. This frequency can easily be sensed. The frequency can also readily be controlled with the aid of a converter. It is also possible to establish the relationship between the frequency and control pressure, or set-point pressure, in the conduit. A control pressure can thus be calculated on the basis of the frequency sensed/detected and the frequency then adjusted to correspond to the control pressure.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.